Merge tag 'v1.21.0rc3' into develop

Synapse 1.21.0rc3 (2020-10-08)
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Bugfixes
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- Fix duplication of events on high traffic servers, caused by PostgreSQL `could not serialize access due to concurrent update` errors. ([\#8456](https://github.com/matrix-org/synapse/issues/8456))

Internal Changes
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- Add Groovy Gorilla to the list of distributions we build `.deb`s for. ([\#8475](https://github.com/matrix-org/synapse/issues/8475))